1. Field of the Invention
This invention relates to the field of subterranean coal fires. More specifically, the invention comprises a method and apparatus for controlling the temperature and oxygenation of a coal fire in order to bring it under control and ultimately extinguish it.
2. Description of the Related Art
Coal remains one of the earth's most important natural resources. A substantial amount of this resource is wasted via the burning of the coal in situ. Coal fires occur in a variety of ways. FIG. 1 shows a coal fire occurring in a seam which intersects the surface. Coal seam 10 slopes upward toward surface 12. A portion of the seam is exposed to the surface. The exposed portion is ignited via a brush fire or other source. Combustion face 14 forms. The combustion face is typically a narrow band of burning coal advancing into the seam.
Collapsed cover 20 falls over the combustion face as it burns. Ground collapse 16 also falls over the combustion face as the support burns away beneath soil/sediment 24. Air 18 is drawn toward the combustion face as the hot combustion products rush upwards. The combustion process itself is often smoldering combustion, since the overlying collapsed cover restricts the oxygen supply.
The rate of combustion typically slows as the combustion face progresses further and further underground. However, the combustion of the seam promotes further grounds collapse and this process generally creates additional ventilation. Thus, the combustion face may continue until it exhausts the seam, encounters the water table, or progresses so deep into the earth that it is starved of oxygen. It is not unusual for such a fire to continue for decades and—in extreme cases—even centuries.
A coal fire in a surface-intersecting seam may be fought conventionally if the fire is detected at its inception (by inundating the exposed portion with water). However, once the fire progresses underground it is very difficult to extinguish. Thus, although the fire starts as a surface fire if it continues it will become a subterranean fire.
Of course, subterranean fires also occur in seams which do not intersect the surface. Such fires are almost always the result of human activity. FIG. 2 shows a portion of a soft-rock subterranean mine. Coal seam 10 lies completely beneath surface 12 (in a layer of soft rock 26). Drift 32 (a horizontal passageway cut to follow the seam) is connected to the surface via ventilation shaft 28. There are typically multiple ventilation shafts in such a mine. There may also be natural vents 30 which connect to the surface.
In the example shown the mining activity has produced a coal fire. Mining involves the use of explosives, the use of arc welding, and other potential ignition sources. Mining may also produce a methane gas explosion and fire which—under certain circumstances—can ignite the coal being mined.
The existing passageways within a mine influence the flow of oxygen and waste gases. In the example of FIG. 2, air is drawn in through natural vent 30 and feeds the combustion process occurring along combustion face 14. Waste gases travel along drift 32 and out ventilation shaft 28. The flow of oxygen and waste gases is generally more complex than is illustrated.
In fighting the fire, efforts are often made to seal the mine so that the oxygen supply will be exhausted. However, most coal mines which are reasonably close to the surface have multiple natural vents. It is often quite difficult to find and cap all the natural vents. Of course, in attempting to eliminate all the oxygen from the mine, one also makes it more difficult for firefighters to work in the mine.
Coal mines are typically much more complex than the example shown in FIG. 2. FIG. 3 shows a plan view of a modestly sized coal mine of the room-and-pillar type. The reader should note that there are many types of coal mines. A description of all the different types of mines is beyond the scope of this disclosure and is—in any event—not necessary for the understanding of the present invention. The room-and-pillar type illustrated in FIG. 3 should therefore be viewed as only one example among many. The inventive methods described subsequently are potentially applicable to all types of mines.
FIG. 3 shows a coal mine 34 positioned to extract coal lying within coal seam boundary 48. Main shaft 36 descends from the surface. A smaller ventilation shaft 38 also descends from the surface. The two shafts are connected via drift 40. The coal removal works outward from drift 40. A number of crosscuts 42 extend perpendicularly from drift 40. Pillars 44 are left between the cross cuts in order to support the roof of the mine.
One or more ventilation bore holes 50 connect portions of the mine to the surface. These are often added as the crosscuts are extended in order to provide suitable ventilation in newly opened parts of the mine. Extraction boundary 46 defines the furthest extent of coal removal. The reader should bear in mind that the extraction boundary is generally being extended as work progresses. In the example of FIG. 3 the extraction process was started on the left side of the view and is working toward the right side.
FIG. 4 shows a sectional elevation view of the mine shown in FIG. 3. Shaft house 52 lies proximate (or over) the entrance to main shaft 36. The shaft house generally contains the hoisting gear which lowers the miners into the mine and extracts the mined material (though material is often extracted instead along a sloped conveyor). Vent shaft 38 is typically covered by a structure which contains ventilating blowers and various controls. Only one level of mining activity is shown. Multiple levels would typically be used to harvest coal from a seam such as is depicted in FIG. 4.
Coal fires are now recognized as a substantial source of greenhouse gas emissions (primarily CO2). They also emit harmful pollutants such as mercury. Recent studies estimate that coal fires produce approximately 3% of all the earth's greenhouse gases. Land lying over such fires may be badly damaged by subsidence. The area around such fires is often rendered uninhabitable via the presence of atmospheric pollutants. Thus, coal fires are a highly destructive phenomenon.
In order to combat a subterranean coal fire, one must first determine its location and extent. There is no issue with detecting coal fires started by mining accidents—at least where the mining activity is licensed activity. However, many subterranean coal fires are started by pit mining in the third world. These fires are generally undocumented.
Detection of subterranean fires may be made by ground level temperature sensors and/or analysis of surface gases. Remote sensing using satellites or aircraft is more difficult. This is true because subterranean coal fires may only raise the surface temperature by 1 or 2 degrees Celsius. Larger variations are typically produced by sunlight versus shadow. However, combinations of surface temperature measurements with accurate subsidence measurements are often able to estimate the extent of a subterranean fire.
Once a fire's perimeter is established, the prior art approach to extinguishment involves (1) reducing the oxygen supply; and (2) drilling bore holes to inundate the fire with water and/or fly ash. Water is inadvisable in controlling fires in which the coal has a significant oxygen content—as the water can actually spread the fire. Fly ash is used for these. Some prior art proposals have also included inundating the fire with liquid nitrogen. The inherent expense of liquid nitrogen has made this approach unattractive. Thus, the prior art approaches have significant drawbacks.
It has long been known to use carbon dioxide to fight relatively small fires. Carbon dioxide could also be used to fight coal seam fires, but this has been impractical in the past owing to containerized carbon dioxide's high cost and limited availability. However, it is expected that carbon dioxide will become cheaper and more readily available in coming years. This will be the result of proposed carbon dioxide capture and storage schemes. Since this gas is now recognized as a type of pollutant (a greenhouse gas) governments around the world—in conjunction with industry—have proposed capturing and storing it instead of releasing it into the atmosphere. The present invention proposes to use carbon dioxide (preferably captured from industrial processes) to fight subterranean coal fires.